1. Field of the Invention
This invention relates to a filter assembly for removing impurities from a fluid and, more particularly, to a filter assembly for removing a wide size range of impurities from a sawmill dip tank solution.
2. Description of Related Art
In the lumber processing industry, it is typical to treat natural lumber with a wood treatment chemical such as a biocide or a fungicide. These wood treatment chemicals are contained in what is known as a "dip" tank. In addition to treatment chemicals, the dip tank also typically includes water. It is common for debris to accumulate in the bottom of the dip tanks. This debris typically varies in size from two inches to several microns and consists of wood chips, sawdust, plastic banding straps, rocks, dirt and any number of other materials common to a sawmill operation.
Presently, there are two ways to deal with the debris that accumulates in the bottom of sawmill dip tanks. The first method is to allow the debris to accumulate in the dip tank over an extended perod of time, such as one year. Subsequently, the tank is drained and the debris is removed by manual labor. This method, however, is becoming environmentally expensive and undesirable. The debris which has been allowed to accumulate in the dip tank will eventually absorb and adsorb a large amount of the treatment chemical and be classified as a hazardous waste. Disposal of this hazardous waste creates environmental problems.
A second method of dealing with debris that accumulates in sawmill dip tanks is to filter the solution contained in the dip tank. The continuous or frequent filtering of the dip tank solution prevents the debris from absorbing and adsorbing a large amount of treatment chemical and being classified as a hazardous waste. Additionally, the debris removed by regular filtering can be used to form wood composites, such as particle board, or even used in pulp wood digesters. Indeed, filtered debris containing a small amount of treatment chemical (i.e., 1000-30,000 ppm) is a useful component in particle board because it controls the growth of bacteria and fungus.
However, prior art systems of filtering the dip tank solution have problems that severely limit their efficiency. Such systems are known to include a sump pump and a canister type filter that is similar to an automotive oil filter. Unfortunately, sump pumps can only handle finely suspended debris and debris of a larger size, such as 1/4 inch, will not pass through the pump. Additionally, the canister type filter does not provide the desired filtering of impurities from the dip tank solution and will repeatedly clog to either prevent effective filtering or to stall the pump.
It is known that a plurality of consecutively arranged filter elements can be used to filter a fluid. For instance, a three stage filtering system consisting of three nested drums lined with blotting paper is disclosed in U.S. Pat. No. 2,865,510. Additionally, U.S. Pat. No. 3,349,919 discloses a system referred to as "series filtering" wherein a plurality of filter elements of different pore size are connected in series and are arranged to receive the fluid to be filtered in an order of decreasing pore size. Such systems, however, do not provide the desired filtering of the solution contained in sawmill dip tanks.